A prior art accelerometer with high performance potential is described in U.S. Pat. No. 3,702,073. The accelerometer comprises three primary components, a reed, and upper and lower stators or magnetic circuits between which the reed is supported. The reed includes a movable paddle that is suspended via flexures to an outer annular support ring, such that the paddle can pivot with respect to the support ring. The paddle, flexures and support ring are commonly provided as a unitary structure composed of fused quartz.
Both upper and lower surfaces of the paddle include capacitor plates and force balancing coils. Each force balancing coil is positioned on the paddle such that the central axis of the coil is normal to the top and bottom surfaces of the paddle, and parallel to the sensing axis of the accelerometer. A plurality of mounting pads are formed at spaced-apart positions around the upper and lower surfaces of the annular support ring. These mounting pads mate with inwardly facing surfaces of the upper and lower stators when the accelerometer is assembled.
Each stator is generally cylindrical, and has a bore provided in its inwardly facing surface. Contained within the bore is a permanent magnet. The bore and permanent magnet are configured such that an associated one of the force balancing coils mounted on the paddle fits within the bore, with the permanent magnet being positioned within the cylindrical core of the coil. Current flowing through the coil therefore produces a magnetic field that interacts with the permanent magnet to produce a force on the paddle. Also provided on the inwardly facing surfaces of the stators are capacitor plates configured to form capacitors with the capacitor plates on the top and bottom surface of the paddle. Thus movement of the paddle with respect to the upper and lower stators results in a differential capacitance change.
In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along the sensing axis results in pendulous, rotational displacement of the paddle with respect to the support ring and the stators. The resulting differential capacitance change caused by this displacement is sensed by a feedback circuit. In response, the feedback circuit produces a current that, when applied to the force balancing coils, tends to return the paddle to its neutral position. The magnitude of the current required to maintain the paddle in its neutral position provides a measure of the acceleration along the sensing axis.
Prior accelerometer designs of the type described above have in general taken two different approaches to the mounting of the reed. In one approach, three mounting pads are equally spaced from one another around the support ring, and the reed is clamped between the stators via such mounting pads. This arrangement results in at least one mounting pad being close to the area of the support ring to which the flexures are attached. Mounting strains and thermal strains are therefore coupled into the flexures, causing bias sensitivities to mounting and temperature. Overall stability of the accelerometer is thus degraded. This design does have the advantage of providing three pads that are widely separated, with at least one pad on each side of the magnetic circuit centerline. Preload forces can therefore be applied at the center of the accelerometer, to provide a stable clamping condition. Thus, this first approach allows for flexibility in preload system design, but results in degraded bias performance.
A second commonly used design approach involves the placement of all three mounting pads to one side of the centerline, as far as possible from the flexure area. As a result, the long support ring section between the mounting pads and the flexures provides isolation of the flexures from thermal strains and mounting strains. This approach also allows two of the mounting pads to be aligned with the centroid of pick-off capacitance, as taught in U.S. Pat. No. 4,182,187. Thus, this mounting pad arrangement results in improved bias performance, but does not allow preloads to be applied at the centerline of the accelerometer, and may degrade alignment stability.